"Good news for Zune 30 users! Following the Nov 13 firmware upgrade, a lot of people are noting an improvement in battery life. To achieve this, we improved the Zune 30 codecs so they are more power-efficient, hence the better battery life. That said, we have seen some reports of a separate battery issue related to the upgrade. This problem occurs when users suspend their Zune hard drives by holding down the play/pause button – in some cases the battery continues to drain. It seems to be affecting only a small percentage of Zune users at this point, but we’re working on including it in our December update (similar to last year) – stay tuned for details and specifics."
Ok, so the good news is that the Zune guys are aware of the problem. Reading between the lines, it seems like if you don't use the Play/Pause off function, and instead let your Zune go to sleep on its own, that might fix the problem. I've been turning on my Zune 30 and letting it go to sleep itself, and haven't had any battery life problems - but it never occurred to me that powering it off manually might be the root cause of the problem.